1. Field of the Invention
This invention relates to a flocculant for water treatment to remove impurities from water and, particularly, to a flocculant containing a polymerized silicic acid solution. The invention relates also to a method for producing the flocculant.
2. Prior Art
It is widely known to use activated silica as a component of a flocculant for water treatment to purify water by removing impurities therefrom. Such activated or polymerized silica, however, has been used as a mere additive to a main flocculant consisting of a metal salt such as aluminum chloride or aluminum sulphate.
Recently, it has been proposed to use a solution of highly polymerized silicic acid as a flocculant for water treatment. For instance, U.S. Pat. No. 4,923,629 discloses a flocculant which comprises a solution containing a highly polymerized silicic acid and a small amount of a metal salt.
One problem of a flocculant comprising a solution of highly polymerized silicic acid is how to increase the concentration of effective components (silicic acid and metal substance) in the solution. Namely, the higher the concentration of effective components in the flocculant solution, the greater the capability of the flocculant for coagulating impurities in water. Moreover, the cost of transporting the flocculant solution can be reduced, because the solution can be stored and transported in a more condensed form. From a practical standpoint, it is desirable for the concentration of SiO.sub.2 in the flocculant solution to be not less than about 8% (by weight) and the concentration of all effective components (SiO.sub.2 +MeO.sub.x) to be not less than 10% (by weight).
It is also well known, however, that the higher the concentration of SiO.sub.2 in a solution of silicic acid, the easier it is for the solution to gel. A gelled solution naturally cannot be used as a flocculant. Therefore, it has not been practiced to use a silicic acid solution in which the concentration of SiO.sub.2 exceeds about 8%. This is a contradictory problem in using a flocculant comprising a silicic acid solution.